A stripline coupler consists typically of a pair of shaped and juxtaposed planar transmission line conductors located between two ground planes. This structure shields the conductors from the electrical influences of adjacent components and helps to contain the electromagnetic fields to the area within the two ground planes. Since the circuit is fabricated with the transmission line conductors contained within and necessarily separated from the two ground planes, provision must be made to connect external components or circuit paths to the internal transmission lines.
Making this connection effectively between the stripline directional couplers and its mother-board in the prior art devices has been one of the factors contributing to a relatively high cost of the devices. Typically, stripline directional couplers have been fabricated by mechanically securing an assembly of multiple boards with stripline paths thereon. Lamination is not typically employed. Drilled hole openings in the outer layers provide a means for effectively allowing soldering to the inner transmission lines. The process is labor-intensive.
The method traditionally used for connecting to the inner striplines is not as reliable as needed and, further, is not cost-effective. However, unit cost reductions are difficult to achieve while at the same time retaining uniform coupling characteristics and a high degree of isolation between certain ones of the ports. Isolation in this context is the ratio of the power of the input to a given port to the power at the isolated port. Isolation is a measure of any unevenness in the substrate or variation in the spacing of the ground planes.
Mounting of these devices to mother-boards with highly reliable, easy-to-effect solder steps, while at the same time conserving on the limited mother-board mounting area, is also desirable, but hard to achieve because of the requirement to fit increasingly larger numbers of components onto mother-boards.
The prior art supplies many techniques for making stripline directional couplers and for effecting exterior connections to the interior striplines. U.S. Pat. No. 4,821,007, issued Apr. 11, 1989, teaches the plating of holes routed through an insulating circuit board and through the interior stripline transmission path. The plated holes then are bisected by cutting the board along a line that passes through each plated hole. The cut forms a semi-cylindrical plated indentation where the cut bisects each hole. These exposed plated surfaces are on the edge of the stripline and, accordingly, can be used as electrical contacts for bead-soldering to a mother-board. Further, since the plated indentation extends to both sides of the cut board, the resulting component can be soldered to the mother-board from either of its sides. While advantage is thus gained from the teaching of 4,821,007 to forming contact pads, the approach has the disadvantage of having to bisect each contact hole which, as noted, may stress the plated area and lead to unreliable connections as well as inefficient use of limited available mounting area.